1. Field of the Invention
The present invention is related to a device which compresses color still image data, for example, in accordance with a JPEG (Joint Photographic Expert Group) algorithm, and records the compressed image data in a recording medium.
2. Description of the Related Art
A standard algorithm, for the encoding of high resolution images and the transfer of that information through telecommunication transmission channels, has been recommended by the JPEG. In order to enable a large-scale data compression, the baseline process of the JPEG algorithm breaks down the original image data into components on a spatial frequency axis using a two-dimensional discrete cosine transformation (two-dimensional DCT) process. Thereafter, the data expressed on the spatial frequency axis is quantized by using a quantization table. The quantized data is then encoded using a Huffman table to generate compressed image data, which are recorded in a recording medium.
When the original image is reproduced from the compressed image data, an inverse process to that described above is performed. Namely, the compressed image data are decoded, are dequantized using the quantization table, and are then subjected to a two-dimensional inverse discrete cosine transformation (two-dimensional IDCT).
Due to carrying out the data compression process, a part of the image data is lost. Accordingly, the quality of the reproduced image obtained by the expansion process is lowered in comparison with the original image. On the other hand, the compression process and the expansion process can then be carried out repeatedly any number of times. However, if the quantization table used in the later compression process is different from that used in the first compression process, the image deterioration is advanced (i.e., gets worse) due to the compression process, even if the amount of data obtained by the compression process is approximately the same.